This invention relates to radiation detectors and, more particularly, to a blocked impurity band detector particularly adaptable for the detection of long wave infrared radiation (LWIR).
In the design of high quality radiation detectors it is a natural goal to make the detector as sensitive as possible to incident radiation within a desired range of frequencies. One well known problem facing designers of high quality radiation detectors is the generic phenomenon known as dark current. This phenomenon is the manifestation of several mechanisms at work within the detector, the net result of which is a flow of detector current in the absence of incident radiation.
The dark current flowing through a detector can appropriately be considered as a source of electrical noise, the magnitude of which bears a direct relationship to the magnitude of the dark current. This dark current noise adversely affects the signal-to-noise ratio of the detector, rendering the detector less sensitive to variations in incident radiation.
One well known dark current mechanism is thermal charge carrier generation. This mechanism acts by freeing donor impurity electrons from their atoms upon the absorption of thermal energy by the semiconductor material. These electrons enter the conduction band, and are swept by an electric field to the positive detector electrical contact. The electric field is created across the detector under normal operation by a voltage potential difference. Such a voltage potential difference is typically applied by an external integrated circuit readout device, such as a hybridized thin film device or a charge coupled device. Additional electrons may be injected into the detector from the negative potential electrical contact of such a readout device. The result of these two mechanisms acting together is the generation of a current through the detector in the absence of incident radiation, or the generation of a dark current.
One well known method to eliminate the thermally induced component of dark current is to cool the radiation detector to within a few degrees of absolute zero. Such a cryogenically cooled detector is, however, difficult to package into a compact, low cost assembly.
Another method which is commonly utilized to reduce dark current is to interpose a relatively high resistance layer between a normally heavily doped, and hence low resistance, detecting layer and one of the electrical contacts of the detector. Such a high resistance layer interrupts the conduction path of the impurity band conduction mechanism, resulting in a reduction in dark current. Hence, the relatively high resistance layer is commonly referred to as a blocking layer and, therefore, a detector utilizing such a layer is known as a blocked impurity band detector.
A particular problem associated with blocked impurity band detectors has been the physical placement of the electrical contacts and the associated readout device. Because of the small dimensions of typical detector arrays, wherein the spacing between individual detectors may be less than 100 microns, conventional wiring interconnection schemes are often impractical. This problem is compounded by the number of individual detector elements contained within an array, a typical value being one thousand or greater.
A solution to this interconnection problem has been the utilization of integrated circuit readout devices, which are fabricated with dimensions comparable to those of the radiation detector. Typically, the individual contacts of the readout device are disposed such that they are in registration with the contacts of the individual detector elements. The detector and readout device are subsequently packaged such that they physically joined together, the readout device thereby making direct electrical contact with the individual detector elements. Thus, it can be appreciated that a radiation detector which is compatible with integrated circuit readout technology has obvious advantages over a detector which is not compatible.
One problem with this type of readout technology, however, is that the fill factor of the detector array may be decreased. Fill factor is a measure of the surface of the array that is available to receive incident radiation. The placement of the electrical contacts and the associated readout device typically results in a reduction of the fill factor of a given array, due to the partial occlusion of the radiation receiving surface.
In some prior detectors known as Blocked Impurity Transducers (BIT), all of the electrical contacts are brought out to the radiation receiving surface. This arrangement placed severe restrictions upon the physical characteristics of the readout circuitry, with the result that such detectors were often incompatible with the integrated circuit readout technology.
In response to the obvious disadvantages created by this type of detector, an alternate form was developed wherein all of the electrical connections are brought out to the surface opposite that of the radiation surface. Commonly known as a Reverse Illuminated Blocked Impurity Transducer (RIBIT), this device is compatible with the integrated circuit readout technology. U.S. Pat. No. 4,507,674, assigned to the assignee of the present invention, is illustrative of this reverse form of detector. The low "fill factor" of the BIT detector is overcome by the RIBIT approach. The fill factor of a RIBIT array can approach unity, and high numbers of hybrid-compatible detectors per focal plane become feasible. However, the RIBIT structure has certain materials and processing-related problem areas which make its production challenging. In the RIBIT structure, the first epitaxial layer must be grown over a bulk silicon substrate which has heavily implanted surface regions to establish backside contact areas. The crystalline and electrical properties of the expitaxial film grown over these regions can be difficult to control and can result in poor detector performance. The RIBIT process also requires a v-groove etch through both epitaxial layers to provide a means of contacting the heavily implanted areas on the substrate. The processing of this v-groove contact may also prove difficult to control.